1. Field of the Invention
The present invention relates to a substrate for circuit wiring on which electronic components and the like are mounted and, more particularly, to a substrate for circuit wiring which is particularly suited for use in an electronic apparatus used in a high-temperature environment (105° C. to 140° C.), and in which provisions are made to prevent the linear thermal expansion of the metallic substrate itself, due to the high temperature, from affecting the mounting condition of the electronic components, etc.
2. Description of the Related Art
For an electronic apparatus incorporating electronic components, in particular, an automotive electronic apparatus mounted in a restricted space subjected to a high-temperature environment, such as an electronic control unit for controlling an automotive engine, etc., there is a need to reduce the size of the apparatus and the space for mounting and, at the same time, increase heat dissipation to enable the electronic apparatus to be mounted in an engine compartment.
As the environment inside the engine compartment is severe because of high temperatures, large variations in temperature, etc., a base substrate made of a ceramic material having high heat resistance has been used for a substrate for circuit wiring on which electronic components are mounted, and a heat sinking construction such as a finned heat sink has been required for heat dissipation. As a result, the electronic apparatus was bulky and, hence, the amount of reduction in size and space is limited. In view of this situation, there has developed a need for a substrate that can achieve reductions in size, space and cost while, at the same time, achieving improvements in heat dissipation performance, and metal-based circuit wiring substrates having excellent heat dissipation characteristics are used to meet such demands.
Traditionally, it has been common to use a highly heat resistant ceramic substrate as the substrate for electronic circuits used in an electronic control unit or the like mounted in a high-temperature environment such as inside an automobile engine compartment. On the other hand, the major reasons that metallic substrates made of aluminum alloy, etc. have come to be used are that the cost of the metallic substrate is lower than that of the ceramic substrate, that the ceramic substrate is brittle and thus intractable, that the ceramic substrate requires the provision of a heat sinking construction, that the metallic substrate can be fabricated in the same process as a conventional resin substrate, and that design changes can be easily implemented in the case of the metallic substrate. For these reasons, the metallic substrate has been used, in particular, for output drive circuitry that does not need high-density packaging.
For reasons of heat dissipation and economy, an aluminum plate is often used for the metal-based circuit wiring substrate; however, when the circuit wiring substrate assembled into an electronic apparatus is repeatedly subjected to heating and cooling, in actual use conditions, a large thermal stress is caused due to the difference in coefficient of thermal expansion between the aluminum plate as the substrate and the electronic components, especially, chip components, mounted thereon. This results in the problem that electrical reliability degrades because, due to the thermal stress, cracks are caused at or near the soldered portions at which the components are fixed.
Furthermore, the metal-based circuit wiring substrate requires the provision of an insulated film on the substrate surface for mounting of the electronic components, as the substrate is constructed from a metallic plate. When a material having low elastic modulus is used for the insulated layer, the stress occurring between the metallic plate and the electronic components can be alleviated. However, as the chip component size increases, the elastic modulus of the material needs to be further reduced significantly, but generally, a low elastic modulus material has poor adhesion to the aluminum and the copper forming the circuit conductors.
The resulting problem is that it is difficult to obtain a metal-based circuit wiring substrate that can achieve excellent adhesion between the circuit wiring substrate and conductive foils such as wiring patterns. In particular, since the adhesion of the conductive foils to the circuit wiring substrate markedly drops under high-temperature conditions, it becomes increasingly difficult to obtain a metal-based circuit wiring substrate having high-temperature resistance that can withstand the use at a temperature exceeding 105° C.
In view of this, there is proposed, for example, in Japanese Unexamined Patent Publication No. 11-8450, a metal-based circuit wiring substrate having excellent heat dissipation characteristics by improving the adhesion between the metallic substrate and the conductive foil pattern, thereby increasing the ability to alleviate the stress so that defects such as cracks will not be caused at or near the soldered portions of electronic components when the metal-based circuit wiring substrate is subjected to rapid heating or cooling.
In the above proposed metal-based circuit wiring substrate, to address the problem of separation in a high-temperature environment, the thermal stress transmitted from the metallic substrate to the electronic components is reduced by reducing the elastic modulus of the insulated layer formed on the metallic substrate. The insulated layer here is formed by adding a rubber-based filler and a silica-based filler to an epoxy resin, thereby lowering elastic modulus while improving the heat dissipation characteristics.
This structure alleviates the stress occurring due to the difference in linear thermal expansion between the metallic substrate and the electronic components mounted thereon, and thus serves to prevent cracks from being caused to the soldered portions of the electronic components. In the prior art, as a silica-based filler was added to the insulated layer to increase heat dissipation, the insulated layer had high elastic modulus.
The prior art metal-based circuit wiring substrate achieves high heat dissipation and low elastic modulus by adding a silica-based filler and a rubber-based filler to the insulated layer; with this structure, the stress occurring due to the difference in linear thermal expansion between the metallic substrate and the electronic components mounted thereon can be alleviated, and thus, cracks can be prevented from being caused in the soldered portions of the electronic components.
At high temperatures, the metallic substrate, the insulated layer, and the electronic components exhibit differing degrees of linear thermal expansion. Accordingly, for the linear thermal expansion of the insulated layer, the proportions of the rubber-based filler and the silica-based filler are adjusted; that is, to alleviate the stress caused by the linear thermal expansion, the linear thermal expansion of the insulated layer is made larger than the linear thermal expansion of the electronic components but held smaller than the linear thermal expansion of the metallic substrate.
However, if the coefficient of linear thermal expansion of the insulated layer is properly adjusted by lowering the elastic modulus of the insulated layer on the metal-based circuit wiring substrate, as the electronic components, the insulated layer, and the metallic substrate respectively have differing coefficients of linear thermal expansion, there arises the problem that the adhesion between the insulated layer and the copper foil wiring pattern as well as the adhesion between the insulated layer and the metallic substrate becomes weak in a high-temperature environment where the structure is repeatedly subjected to high and low temperatures and, in the case of a low elastic modulus material, the possibility of the aluminum plate and the circuit conductors of copper becoming separated from the insulated layer increases because adhesion of the low elastic modulus material to the aluminum and the copper is poor.
It is accordingly an object of the present invention to provide a substrate for circuit wiring which is particularly suited for use in an electronic apparatus used in a high-temperature environment exceeding 105° C., and in which the linear thermal expansion of the metallic substrate itself is prevented from affecting the mounting condition of electronic components, etc.; this is achieved by molding the metal-based circuit wiring substrate with a resin material whose coefficient of linear thermal expansion has been adjusted.